1. Technical Field
The present disclosure relates to a photodetection device including an interference element.
2. Description of the Related Art
A device that measures the shape or a distance of an object with high accuracy and in a noncontact manner by utilizing an interference phenomenon of light is in practical use. Generally in such a device, the light that has reflected off or passed through an object, which is referred to as an object light, and the light that has reflected off a reference surface, which is referred to as a reference light, are caused to interfere with each other. The generated interference light is imaged and observed. In a state where the degree of the flatness of the reference surface is sufficiently ensured, interference fringes of the interference light occur according to the optical path length of the object light. Difference in optical path length that corresponds to the wavelength of the light causes interference fringes of one period. Thus, the three-dimensional shape of a measured surface of the object can be determined from the pattern of the interference fringes.
The difference in optical path length equal to or larger than the wavelength of the light causes a repeat of interference fringes. When the measured surface of the object is smooth, the difference in the optical path length beyond the wavelength of the light can be estimated by counting these interference fringes.
When the measured surface of the object includes a step beyond the wavelength, the interference fringes lack in the step portion and accordingly, the difference in the optical path length is unable to be determined accurately. As a method of measuring the shape of an object in such a case, two-wavelength interferometry is known. The two-wavelength interferometry is described in for example, Japanese Unexamined Patent Application Publication No. 10-221032 and Yeou-Yen Cheng and James C. Wyant: “Two-wavelength phase shifting interferometry”, Applied Optics, vol. 23, No. 24, pp. 4539 to 4543.
The two-wavelength interferometry uses lights with two wavelengths to perform interference measurement. Images of the interference fringes according to the lights with the respective wavelengths are independently or simultaneously picked up and on the basis of information on the interference fringes of both of the wavelengths, the shape of the measured surface of the object is determined. When the two wavelengths are referred to as λ1 and λ2, it is known that an effective measurement wavelength λeff is obtained by the two-wavelength interferometry as expressed below.
      λ    eff    =                    λ        1            ⁢              λ        2                                            λ          1                -                  λ          2                          
When for example, λ2=1.1×λ1, λeff=11×λ1 and a step that is larger can be accurately estimated.